Software-defined networking (“SDN”) has gained momentum among providers of network services, including data centers, cloud computing, and wide-area networks (“WANs”) service providers. SDN enables network service providers to utilize network resources more efficiently by decoupling the control plane from the data plane and introducing a logically centralized controller. SDN networks facilitate more efficient routing of traffic flows using a centralized network view. On the other hand, traditional distributed routing still enjoys the advantage of better scalability, robustness, and swift reaction to events such as failures. There are therefore significant potential benefits to adopting a hybrid operation where both distributed and centralized routing mechanisms co-exist. Furthermore, two mechanisms operating simultaneously increases robustness since even if one of mechanism fails, the other mechanism will continue to operate. The advantages of adopting a centralized controller are accompanied by implementation challenges with regard to compatibility since not all devices support full SDN functionality and scalability since the centralized controller can be overloaded when the network scales beyond its computational capacity.
Network service providers, such as Internet service providers (“ISPs”) that currently operate well-functioning networks, have additional concerns regarding migration to an SDN network. One such concern is the cost of converting legacy networks to SDN networks, which requires extensive hardware investments to install SDN-compatible devices and systems. Network service providers also are wary of migrating from legacy networks to SDN networks because of concerns about network robustness of new SDN networks when the legacy networks operate with networking technologies proven to be robust. Moreover, unlike distributed control, centralized control suffers from decision delay because decisions are made remotely. To respond quickly to condition changes, the centralized controller should be positioned close to its control targets (e.g., local routers), which limits its applicability to WANs. These factors justify why many ISPs have decided to implement hybrid SDN networks in which legacy distributed routing mechanisms remain functional alongside centralized control provided by a centralized controller.
The dual control paradigm in a hybrid SDN network creates new challenges for network management, including routing stability and reconciliation of imperfect information. Stability is a fundamental concern in network routing. A stable routing mechanism maintains the same route for the same traffic flow as long as possible. In the presence of multiple routing control units, a stable route is the route that remains unchanged by any other routing units. Imperfect information poses another challenge. Perfect information about a network's topology and the network's available bandwidth is critical to routing performance. Delayed routing convergence can occur when information about the network topology is obsolete. In a hybrid SDN network, the centralized controller must carefully monitor the network because conflicting views between the centralized controller and distributed local routers can result in disagreements about routing decisions, which can directly cause route flapping and instability.